Generally, an olefin polymer is produced by polymerization in the presence of a Ziegler-Natta catalyst containing a titanium compound and an organic aluminum compound. For example, in the production of a polypropylene that is one of olefin polymers, an isotactic polypropylene is obtained mainly in the presence of a catalyst containing a solid catalyst component formed mainly from titanium, magnesium, chlorine and an electron-donating compound and containing an organic aluminum compound as a co-catalyst component and an organosilicon compound having an alkoxy group as a stereoregularity improver. Attempts are presently made to attain an improvement in the catalytic activity during polymerization, an improvement in stereoregularity of an olefin polymer, an improvement in the form of a polymer powder for stable production of an olefin polymer and a reduction in residual Cl in the polymer.
When the above residual Cl in a polymer is large in amount, for example, the Cl corrodes a mold for injection molding, a polymer absorbs water during the production of a biaxially oriented film or its spinning to cause foaming, or a foreign matter containing an additive is formed, which makes high-speed molding difficult, so that it is a big issue to decrease the amount of residual Cl in a polymer.
As means for overcoming the problem of residual Cl in a polymer, first, it is general practice to employ a method in which a catalyst is improved in activity. In the second place, it is also general practice to employ a method in which a non-organic material such as silica is allowed to support a magnesium compound to substantially decrease a magnesium chloride content, i.e., a Cl content in a catalyst.
For example, there is known a method in which silica, butyl octyl magnesium and hydrogen chloride gas are brought into contact to form a magnesium-chloride-supporting silica support, the thus-prepared support is treated with an alcohol and then the support is allowed to carry titanium tetrachloride and an electron-donating compound (JP-A-63-280707) or a method in which a mixture of silicon tetrachloride with silane trichloride is brought into contact with a product prepared by bringing silica and butyl octyl magnesium into contact, then, the resultant mixture is washed with an inert solvent to form a magnesium-chloride-supporting silica support, and the thus-prepared support is treated with an alcohol and then allowed to carry titanium tetrachloride and an electron-donating compound (Japanese Specification Publication No. 4-506833 of PCT Application).
Further, there is also known a method in which silica, butyl ethyl magnesium and ethanol are brought into contact to form a magnesium-ethoxide-supporting silica support, the thus-formed support is reacted with silicon tetrachloride and then washed with heptane, and the thus-prepared support is further reacted with an electron-donating compound at 50° C. and with titanium tetrachloride at 90° C. once each (JP-A-61-174206), a method in which silica is impregnated with a mixture of magnesium chloride with butanol to form a silica support supporting a butanol complex of magnesium chloride and the silica support is allowed to carry titanium tetrachloride and an electron-donating compound (JP-A-63-168413), or a method in which silica pre-treated with trimethyl chlorosilane is allowed to carry diethoxymagnesium and then allowed to carry titanium tetrachloride and an electron-donating compound (JP-B-7-17695).
However, while olefin polymers obtained by the above methods have certain performances, particularly, the above methods or the olefin polymers are not fully satisfactory in polymerization activity, stereoregularity, residual Cl, and the like.
On the other hand, as a method of improving olefin polymers in the morphology including a particle diameter and a form, JP-A-58-000811 discloses a method in which a magnesium compound is once dissolved in a solvent such as an alcohol and then re-precipitated and the thus-obtained precipitate is used.
In the above method, however, it is essential to carry out the procedures of supporting, dissolving and precipitation of a magnesium compound, so that there are defects that steps thereof are complicated and that a catalyst is poor in stability of performances. Further, the above method also has another defect that the catalyst activity during polymerization and the stereoregularity of an olefin polymer are not sufficient.
As a method of overcoming the above defects, therefore, JP-A-2-02-413883 discloses a method in which metal magnesium, an alcohol and a specific amount of a halogen reaction product are used as a support for a catalyst, and JP-B-07-025822 discloses a method of producing an olefin polymer in the presence of a Ziegler-Natta catalyst containing a solid catalyst component obtained by adding an organic acid ester to a reaction product from alkoxymagnesium, a halogenating agent and alkoxytitanium and further reacting a titanium halide with the resultant mixture.
Further, there is known a method of producing an olefin polymer in the presence of a solid catalyst component obtained by suspending diethoxymagnesium in alkylbenzene, reacting the diethoxymagnesium with predetermined amounts of titanium tetrachloride and phthalic acid diester at a temperature of 80° C. or higher but 120° C. or lower, to obtain a solid substance, washing the solid substance with alkylbenzene and reacting the solid substance with a predetermined amount of titanium tetrachloride in the presence of alkylbenzene (JP-A-64-69608)
In these methods, however, the catalytic activity during polymerization and the stereoregularity of olefin polymers are not yet sufficient.
Further, JP-A-11-269218 discloses a solid catalyst component for olefin polymerization, obtained by bringing a magnesium compound and a titanium compound into contact with each other in the presence of an electron-donating compound at a temperature of 120° C. or higher but 150° C. or lower and then washing the reaction mixture with an inert solvent at a temperature of 100° C. or higher but 150° C. or lower. There are produced effects that a decrease in the catalytic activity with the passage of time during polymerization is suppressed and that an olefin polymer is improved in stereoregularity.
Since, however, the polymerization activity of the above catalyst is not necessarily fully satisfactory, the catalyst needs a further improvement in this activity.
It is an object of the present invention to provide a solid catalyst component for olefin polymerization, which has high polymerization activity and which gives an olefin polymer excellent in stereoregularity, residual Cl and the state of a powder, a catalyst for olefin polymerization and a process for producing an olefin polymer.
For achieving the above object, the present inventors have made diligent studies and as a result have found that a solid catalyst component for olefin polymerization, of which the residual alkoxy-group content is remarkably decreased, can be obtained by a specific production method, and that the above problems can be thereby overcome. The present invention has been accordingly completed.